Light-emitting device and display apparatus

ABSTRACT

The object of the present invention is to provide a light-emitting device that emits phosphorescence with high efficiency. The light-emitting device of the present invention includes a host containing two kinds of dopants, wherein a dopant having a longer maximum-emission wavelength is doped in a low concentration and has a substituent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device containing aphosphorescent metal complex having an organic ligand, and a displayapparatus including the light-emitting device.

2. Related Background Art

There has been known an organic electroluminescence device (hereinafter,also referred to as “organic EL device”) as a light-emitting device.

The organic EL device has at least one organic layer provided betweenelectrodes. An organic layer may have a multi-layer structure, forexample, a three-layer structure including a hole transporting layer, alight-emitting layer, and an electron transporting layer, or afour-layer structure including a hole transporting layer, alight-emitting layer, an exciton diffusion blocking layer, and anelectron transporting layer.

The hole transporting layer may be composed of α-NPD. The light-emittinglayer may be composed of Alq3, CBP as a host doped with Ir(ppy)₃ or CBPas a host doped with PtOEP. The exciton diffusion blocking layer may becomposed of BCP. The respective compounds are listed below in full.

Alq3: aluminum-quinolinol complex

α-NPD: N4,N4′-Di-naphthalen-1-yl-N4,N4′-diphenyl-biphenyl-4,4′-diamine

CBP: 4,4′-N,N′-dicarbazole-biphenyl

BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline

PtOEP: platinum octaethylporphyrin complex

Ir(ppy)₃: iridium-phenylpyridine complex

The structures of the above compounds except PtOEP are shown in FIG. 3.

Also, an electron transporting material includes oxadiazole derivatives.

Examples of an organic EL device making use of phosphorescence arereported in, for example, “Improved Energy Transfer inElectrophosphorescent Device” (D. F. O'Brien et al., Applied PhysicsLetters Vol. 74, No. 3, p. 422 (1999)) and “Very High-efficiency GreenOrganic Light-emitting Devices based on Electrophosphorescence” (M. A.Baldo et al., Applied Physics Letters Vol. 75, No. 1, p. 4 (1999)).

The technique of converting an excitation energy from triplet excitationto singlet excitation is disclosed by U.S. Pat. No. 6,310,361, AppliedPhysics Letters: 79, 7, 1045 (2001), and Nature: Vol. 403, 750 (2000).

Besides, Japanese Patent Application Laid-Open No. 2003-077674 disclosesthat an energy is converted from a triplet exciton to triplet excitationand light is emitted from the triplet excitation.

However, none of the above techniques offer a satisfactory device interms of phosphorescence efficiency, device lifetime, and powerconsumption.

SUMMARY OF THE INVENTION

The present invention has been made in light of solving theabove-mentioned problem and therefore has an object to provide alight-emitting device that emits phosphorescence and is adequate forpractical use.

Therefore, the present invention provides a light-emitting devicecomprising: a pair of electrodes; and a light-emitting layer providedbetween the pair of electrodes, wherein the light-emitting layer has ahost and two kinds of dopants, and wherein each of the two kinds ofdopants is a phosphorescent metal complex having an organic ligand, andamong the two types of dopants, a dopant having a longermaximum-emission wavelength has a substituent in a ligand structure andis contained in the light-emitting layer at a lower concentration thanthe concentration of the other dopant.

According to the present invention, a light-emitting device emittingphosphorescence can be provided, which is adequate for practical usewith a high efficiency, long lifetime, and low power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic sectional views showing two kinds oflight-emitting devices according to the present invention;

FIG. 2 is a graph showing an emission spectrum;

FIG. 3 shows chemical structure of Alq3, α-NPD, CBP, BCP and Ir(ppy)₃.

FIG. 4 shows chemical structure of the organic compounds used in thelight-emitting device of the present invention;

FIG. 5 shows chemical structure of the organic compounds used in thelight-emitting device of the present invention; and

FIG. 6 shows chemical structure of the organic compounds used in thelight-emitting device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The light-emitting device of the present invention is a light-emittingdevice comprising a light-emitting layer having a host and two kinds ofdopants, wherein each of the two kinds of dopants is a phosphorescentmetal complex having an organic ligand, and wherein among the two typesof dopants, a dopant having a longer maximum-emission wavelength has asubstituent in a ligand structure and is contained in the light-emittinglayer at a lower concentration than the concentration of the otherdopant.

The term “light-emitting layer” used herein refers to a layerfunctioning to emit light among organic layers interposed betweenelectrodes. The term “host” means a main component in the light-emittinglayer. Thus, the term “dopant” means a component slightly contained inthe light-emitting layer.

In the present invention, each of two dopants can emit phosphorescencein the absence of the other dopant.

The light-emitting layer may contain not only the two kinds of dopantsbut also any other dopants. In the presence of any other dopants, energytransfer is efficiently carried out between the two kinds of dopants anda low-concentration dopant emits light with a longer wavelength.

The light-emitting layer may contain another component in addition tothe host and dopant.

In the light-emitting device, the dopant which emits light with at leasta longer wavelength (dopant with a longer maximum-emission wavelength)emits light among the two kinds of dopants. Of course, each of thedopants can emit light, but the dopant emitting light with a longerwavelength preferably emits light for main light emission. The term“main light emission” refers to emission from a dopant emitting lightwith a longer wavelength as the maximum-wavelength at an emissionspectrum peak which can be observed from the light-emitting device. Inthe presence of two dopants different in wavelength, a spectrum peak ofthe two kinds of dopants in combination at the maximum emissionintensity may fail to match with their respective spectrum peaks at thetime when the two kinds of dopants emit light independently of eachother, and slightly deviate therefrom. In the present invention, it ispreferable to adopt light emission spectrum peak of those spectrumpeaks, which is closer to the emission spectrum peak of the dopantemitting light with a longer wavelength, and is farther from an emissionspectrum peak of the dopant (the other dopant) emitting light with ashorter wavelength. Based on the above, the light-emitting device of thepresent invention will be regarded as one capable of performing lightemission from the dopant emitting light with a longer wavelength.

A concentration of the dopant means a ratio of dopants contained in thelight-emitting layer to all components in the light-emitting layer. Theunit of the concentration is expressed by “wt. %”.

The dopant emitting light with a longer wavelength has a substituent ina ligand structure. The substituent can thus reduce an intermolecularinteraction and control an intermolecular quenching mechanism.

With such a structure, the energy transfer to the dopant emittingphosphorescence from the other dopant capable of emittingphosphorescence (assist dopant) can be efficiently carried out. Further,it is possible to control intermolecular quenching with the dopantemitting phosphorescence, resulting in improved emission efficiency.Thus, a light-emitting device that attains longer lifetime and low powerconsumption can be provided as the light-emitting device.

In the present invention, the two kinds of dopants are preferablycomplexes having similar structures. The term “similar structures”refers to structures having the same central metal or structures havinga substituent or no substituent where one dopant with the longermaximum-emission wavelength has a structure of the other dopant(unsubstituted) and a substituent thereon. The central metal isparticularly preferably iridium, but may be platinum, copper, orrhenium. Ligands of both dopants are preferably 1-phenylisoquinoline. Asanother preferable example, at least the other dopant has a ligand ofbenzoquinoline.

In the present invention, preferably, a phosphorescence lifetime of atleast the main light-emitting material (dopant with the longermaximum-emission wavelength) is as short as 1.6 μs or less.

Also, in the present invention, preferably, a peak difference betweenthe emission spectrum peaks specific to the main light-emitting materialand an auxiliary light-emitting material (the other dopant) is 30 nm orshorter. With such a relation, the energy transfer between the twodopants can be facilitated and/or even with the two kinds of dopants, itis possible to visually reproduce color of main light emission.

Also, in the present invention, preferably, the auxiliary light-emittingmaterial (the other dopant) has structural isomers both of which arepreferably contained in the light-emitting layer. Such a structureprovides the broad emission wavelength range and facilitates the energytransfer to the main light-emitting material and/or the presence of thestructural isomers having the same property prevents crystallization inthe light-emitting layer.

At least one of the electrodes is preferably transparent. Also, theelectrode may be suitably selected while considering electron injectionproperty and hole injection property. Examples of the transparentelectrode material include ITO and IZO. The other electrode may becomposed of aluminum, gold, platinum, chromium, copper or the likealone, or may be one containing at least one of those elements.

It is also preferable that the light-emitting device be designed to beprotected from oxygen or moisture if necessary. For example, thelight-emitting device may be provided with a sealing can or sealing filmcontaining either an organic material or an inorganic material.

In the present invention, the light-emitting device can be used for adisplay apparatus such as a display, for example, for a pixel unit orsub-pixel unit of the display. The term “display” refers to a displayapparatus installed in a television, a personal computer, a digitalcamera, a camcorder, etc., or refers to a display apparatus mounted to avehicle body. Alternatively, the light-emitting device can be used as anilluminator or a display unit of an image forming apparatus employing anelectrophotographic process or an exposure light source for aphotosensitive member.

The light-emitting device may be used alone or a plurality oflight-emitting devices may be used in combination. In the case of usingthe plurality of the devices, the devices may be driven in a passive oractive matrix manner to emit light. Also, in such a case, the respectivedevices may be identical or different in emission light color. In thecase of using the plurality of the devices different in emission lightcolor, it is possible to carry out full-color emission. Also, thelight-emitting device may have a so-called bottom emission structurewherein light can be received from a substrate side or a top emissionstructure wherein light can be received from the side opposite to thesubstrate.

As shown in FIGS. 1A and 1B, the light-emitting device may have threeand four organic layers, respectively. FIGS. 1A and 1B are schematicsectional views each showing the light-emitting device according to thepresent embodiment. Reference numeral 1 denotes a metal electrode; 2, anelectron transporting layer; 3, a light-emitting layer; 4, a holetransporting layer; 5, a transparent electrode; 6, a transparentsubstrate; and 7, an exciton diffusion blocking layer.

Next, organic compounds used in the light-emitting device of the presentinvention will be shown in FIGS. 4, 5 and 6. In particular, a dopantemitting red light is shown. However, the present invention isapplicable to emission in any of three primary colors, i.e., red, green,and blue, or to emission in neutral color.

A host material used in the light-emitting layer includes CBP or TAZ,the main light-emitting material includes Ir(4F5 mpiq)₃ or Ir(4mopiq)₃,and the unsubstituted non-light-emitting material includes Ir(bq)₃ orIr(piq)₃. Combination examples are summarized in Table 1.

Emission Emission First dopant wavelength Second dopant wavelengthLigand (nm) Ligand (nm) Ir(bq)₃ 585 Ir(4Fpiq)₃ 605 Ir(4F5mpiq)₃ 610Ir(4mopiq)₃ 610 Ir(4mpiq)₃ 615 Ir(C6piq)₃ 615 Ir(4CF3piq)₃ 605Ir(popiq)₃ 610 Ir(4Fp5Fiq)₃ 605 Ir(4F5Mp5Fiq)₃ 615 Ir(4CF3p5Fiq)₃ 610Ir(4Fp4Fiq)₃ 610 Ir(ppy)₃ 510 Ir(5Fppy)₃ 525 Ir(P-4CF3-py)₃ 540Ir(5CF3-ppy)₃ 520 Ir(dbq)₃ 510 Ir(5Fppy)₃ 525 Ir(P-4CF3-py)₃ 540Ir(5CF3-ppy)₃ 520 Ir(piq)₃ 620 Ir(C8piq)₃ 620 Ir(C2piq)₃ 620 Ir(5Fpiq)₃625 Ir(5ppiq)₃ 625 Ir(Fliq)₃ 650 Ir(p5Fiq)₃ 625 Ir(C4p5Fiq)₃ 625Ir(4F5Mp4Fiq)₃ 620

In the following examples, as a device structure, a device having threeorganic layers as shown in FIG. 1A was used. FIG. 1A is a schematicsectional view of a light-emitting device according to the presentexamples. Reference numeral 1 denotes the metal electrode; 2, theelectron transporting layer; 3, the light-emitting layer; 4, the holetransporting layer; 5, the transparent electrode; and 6, the transparentsubstrate.

ITO was patterned and deposited to a thickness of 100 nm on a glasssubstrate as the transparent substrate 6 to prepare the transparentelectrodes 5 having an electrode area of 3 mm². The following organiclayers and electrode layer were continuously formed by vacuumevaporation using resistance heating in a vacuum chamber at a vacuumdegree of 10⁻⁵ Pa.

Hole transporting layer (40 nm): FL03

Light-emitting layer (60 nm): CBP (host)+predetermined two kinds ofphosphorescent materials (dopants)

Electron transporting layer (50 nm): Bphen

Metal electrode layer 1 (10 nm): KF

Metal electrode layer 2 (100 nm): Al

Example 1

CBP was used as a host of the light-emitting layer, and thelight-emitting layer was doped with

Ir(bq)₃ as a short-wavelength light-emitting material unsubstituted in aconcentration of 8 wt. % and with Ir(4mopiq)₃ as a long-wavelengthlight-emitting material in a concentration of 4 wt. % to thereby preparea light-emitting device. The obtained light-emitting device was measuredfor an efficiency (lm/W at 600 cd/m²), chromaticity (x, y) (measuringdevice: BM-7 available from TOPCON Co.), lifetime (luminance half-timeat the time of driving at 100 mA/cm² (hr)), and current amount (at thetime of applied voltage: 8V). Table 2 shows the result of measurements.

Examples 2 to 5

The short-wavelength light-emitting material unsubstituted: Ir(bq)₃ andthe long-wavelength light-emitting material: Ir(4mopiq)₃ were doped indifferent concentrations. Table 2 shows their respective concentrationsand evaluation results of light-emitting device characteristics.

Comparative Examples 1 to 3

The short-wavelength light-emitting material unsubstituted: Ir(bq)₃ andthe long-wavelength light-emitting material: Ir(4mopiq)₃ were doped indifferent concentrations. Table 2 shows their respective concentrationsand evaluation results of light-emitting device characteristics.

The light-emitting layer of each of Examples 1 to 6 of the presentinvention and Comparative Examples 1 to 3 had the structural isomers ofIr(bq)₃. It was confirmed that facial: meridional=95:5.

TABLE 2 Chromaticity Current amount Doping CIE Lifetime (hr) (mA/cm²)concentration (%) Efficiency coordinates Luminance Applied Ir(bq)₃Ir(4mopiq)₃ lm/W Evaluation (x, y) Evaluation half-time Evaluation with8 V Evaluation Example 1 8 4 10.3 ∘ 0.65, 0.35 ∘ 40 ∘ 144 ∘ Example 2 74 10.1 ∘ 0.65, 0.35 ∘ 26 Δ 118 ∘ Example 3 10 2 12.2 ∘ 0.63, 0.36 Δ 26 Δ88 ∘ Example 4 10 5 10 ∘ 0.65, 0.35 ∘ 34 ∘ 92 ∘ Example 5 12 3 11.8 ∘0.64, 0.36 ∘ 23 Δ 282 ∘ Comparative 6 1 10.5 ∘ 0.62, 0.37 x 23 Δ 69 xExample 1 Comparative 10 0 21.4 ∘ 0.52, 0.47 x 35 ∘ 189 ∘ Example 2Comparative 0 10 7.3 Δ 0.66, 0.34 ∘ 11 x 66 x Example 3

In Table 2, the mark “∘” ranks as the most adequate for practical use,and the ranking order is ∘>Δ>x.

Considering the efficiency, efficiency of a device of ComparativeExample 3 that did not contain an assist dopant, i.e., Ir(bq)₃ was usedas a reference (represented by the mark “Δ” in Table 2) and efficiencyof 10 lm/W or more was judged as the mark “∘”.

As regards the current amount, a low voltage is preferred in the case ofconstant-current driving. Hence, a reference voltage was set to 8V, anda current amount was measured and evaluated. When a current density of70 mA/cm² is obtained at the reference voltage of 8 V, a drive voltagenecessary for obtaining brightness of about 300 cd/m² with VGA of a 0.3mm²-pixel can be preferably set to around 5 V. Therefore, the value of700 mA/cm² or more was judged as the mark “∘” and the value below 700mA/cm² was judged as the mark “x”.

Regarding the lifetime, the luminance half-time was obtained as a resultof a 50-fold acceleration test and thus, in terms of actual time, thelifetime less than 1,000 hours was judged as the mark “x”, the lifetimeof 1,000 hours or more was judged as the mark “Δ”, and the lifetime of1,500 hours or more was judged as the mark “∘”.

Regarding the chromaticity, a value deviated by 6% or less from an NTSCstandard (red chromaticity coordinates: x=0.68, y=0.32) was used as areference, the chromaticity with x less than 0.63 was judged as the mark“x”, the chromaticity with x equal to 0.63 was judged as the mark “Δ”,and the chromaticity with x of 0.64 or more and y of 0.34 or more wasjudged as the mark “∘” for red light emission. Here, the mark “Δ”represents the lowest acceptable value and a range of the mark “∘” ispreferably used.

The current amount and lifetime vary depending on an amount of theassist dopant. In these example, Ir(bq)₃ was used as the assist dopant.It was revealed that due to its concentration change, the light-emittingdevice characteristic was changed at a certain concentration. Morespecifically, the lower limit is 7 wt. % and the upper limit is 12 wt.%. A preferable concentration range is 8 wt. % or more and 10 wt. % orless. The evaluation results of the current amount and the lifetime, andthe assist dopant concentrations are summarized in Table 3 below.

TABLE 3 Doping concentration of Ir(bq)₃ Concentration 0% 6% 7% 8% 10%12% Current x x ∘ ∘ ∘ ∘ Lifetime x x Δ ∘ ∘ Δ

From the above, it was found that the efficiency and chromaticity varieddepending on the content of the dopant performing the main lightemission. The specific amount is 2 wt. % or more and 10 wt. % or less.In the present examples, in order to emit light of red as a primarycolor, the evaluation result that the doping concentration ofIr(4mopiq)₃ was 1 wt. % is judged as the mark “x”. However, theconcentration of the dopant as the main light-emitting material will bepreferably less than 2 wt. %, for example, 1 wt. % insofar as a desiredchromaticity can be obtained. In this case, the emission light becomeslight close to orange light rather than red light. The evaluationresults of the efficiency and the chromaticity, and the concentrationsof the dopant as the main light-emitting material are summarized inTable 4 below.

TABLE 4 Doping concentration of Ir(4mopiq)₃ Concentration 0% 1% 2% 3% 4%5% 10% Efficiency ∘ ∘ ∘ ∘ ∘ ∘ Δ Chromaticity x x Δ ∘ ∘ ∘ ∘

FIG. 2 is a graph showing emission spectrum (Ir(bq)₃+Ir(4mopiq)₃) in thecase of a light emitting device in which the light-emitting layercontains Ir(4mopiq)₃ as the dopant for the main light emission, andIr(bq)₃ as the assist dopant; an emission spectrum (Ir(4mopiq)₃) in thecase of the device in which Ir(4mopiq)₃ solely emits light; and anemission spectrum (Ir(bq)₃) in the case of the device in which Ir(bq)₃solely emits light. In the graph, λmax of Ir(4mopiq)₃ is 610 nm and λmaxof Ir(bq)₃ is 585 nm. As apparent from the graph, the emission spectrumof Ir(bq)₃+Ir(4mopiq)₃ is substantially the same as the emissionspectrum of Ir(4mopiq)₃ and their maximum emission wavelength peaks aresubstantially the same.

Further, it was confirmed that as for Ir(4mopiq)₃ doped at the higherconcentration, an effect was obtained at a concentration of 10% or lessfrom the viewpoint of efficiency (lm/W).

Example 6

CBP was used as the host of a light-emitting layer, and thelight-emitting layer was doped with Ir(bq)₃ as a short-wavelengthlight-emitting material unsubstituted and Ir(4F5 mpiq)₃ as along-wavelength light-emitting material in concentrations of 11 wt. %and 3 wt. %, respectively, to thereby prepare a light-emitting device.

Table 5 shows an efficiency (lm/W at 600 cd/m²) and a luminancehalf-time (hr) at the time of driving at 100 mA/cm²

Comparative Examples 4 and 5

In Comparative Example 4, a light-emitting device was prepared similarlyto Example 6 except that Ir(4F5 mpiq)₃ was doped alone in aconcentration of 10 wt. %. In Comparative Example 5, Ir(4F5 mpiq)₃ wasused as the short-wavelength light-emitting material, Ir(piq)₃ was usedas the long-wavelength light-emitting material, and the concentrationsof the respective dopants were both set to 10 wt. %. The devices wereevaluated as in Example 6. The evaluation results of those devices areshown in Table 5.

TABLE 5 lm/W Half-time (hr) Example 6 11.0 40 Comparative Example 4 7.31 Comparative Example 5 4.9 2.2

Spectrum measurement (measuring device: SR-1 available from TOPCON Co.)revealed that the main light-emitting material in Example 6 andComparative Example 4 was Ir(4F5 mpiq)₃, and the main light emittingmaterial in Comparative Example 5 was Ir(piq)₃.

As apparent from Table 5, doping CBP with the mixture of Ir(4F5 mpiq)₃and Ir(bq)₃ improved efficiency rather than doping CBP with Ir(4F5mpiq)₃ alone, and in addition, a main light-emitting material having asubstituent showed effects.

Examples 7 and 8, and Comparative Examples 6 and 7

CBP was used as the host of a light-emitting layer, and thelight-emitting layer was doped with Ir(bq)₃ as the short-wavelengthlight-emitting material unsubstituted in a concentration of 10 wt. % anddoped with Ir(4mopiq)₃ as the long-wavelength light-emitting material inconcentrations of 1 wt. % and 3 wt. % to thereby prepare light-emittingdevices (Examples 7 and 8). CBP was used as the host of a light-emittinglayer, and the light-emitting layer was doped with Ir(bq)₃ as theshort-wavelength light-emitting material unsubstituted in aconcentration of 10 wt. % and doped with Ir(piq)₃ as the long-wavelengthlight-emitting material in concentrations of 1 wt. % and 3 wt. % tothereby prepare ligt-emitting devices (Comparative Examples 6 and 7).Table 6 shows a relation between the concentration of thelong-wavelength light-emitting material and the efficiency (lm/W) in theabove case.

TABLE 6 1% 3% Ir(4mopiq)₃ 19.7 cd/A (Example 7) 17.2 cd/A (Example 8)Ir(piq)₃   12 cd/A (Comparative  7.7 cd/A (Comparative Example 6)Example 7)

It was revealed from Table 6 that an iridium complex having asubstituent was less likely to decrease its efficiency at a higherconcentration. Ir(piq)₃ having no substituent decreased its efficiencydown to 60% when its concentration was changed from 1% to 3%, whileIr(4mopiq)₃ having a substituent could keep its efficiency at 90% whenits concentration was changed from 1% to 3%.

This application claims priority from Japanese Patent Applications No.2003-305851 filed Aug. 29, 2003 and No. 2004-225630 filed Aug. 2, 2004,which are hereby incorporated by reference herein.

1-9. (canceled)
 10. A light-emitting device comprising: (a) a pair ofelectrodes; and (b) a light-emitting layer provided between the pair ofelectrodes, wherein the light-emitting layer has a host, a first dopantand a second dopant, and wherein (i) each of the first and seconddopants is a phosphorescent metal complex having an organic ligand, (ii)a maximum-emission wavelength of the first dopant is longer than amaximum-emission wavelength of the second dopant, (iii) an amount of thefirst dopant present in the light emitting layer is less than an amountof the second dopant present in the light emitting layer, and (iv) thesecond dopant is contained in the light-emitting layer in an amount of7% by weight or more based on the total weight of the light-emittinglayer.
 11. A light-emitting device according to claim 10, wherein thefirst dopant is a phosphorescent metal complex represented by thefollowing formula:

and the second dopant is a phosphorescent metal complex represented bythe following formula:


12. A light-emitting device according to claim 10, wherein the seconddopant transfers energy to the first dopant.
 13. A light-emitting deviceaccording to claim 12, wherein a difference between the maximum-emissionwavelength of the first dopant and the maximum-emission wavelength ofthe second dopant is 30 nm or less.
 14. A light-emitting deviceaccording to claim 12, wherein the second dopant is composed ofstructural isomers.
 15. A light-emitting device according to claim 10,wherein at least one of the pair of electrodes is a transparentelectrode.
 16. A light-emitting device according to claim 15, whereinthe transparent electrode is ITO or IZO.
 17. A digital camera on which alight-emitting device according to claim 10 is mounted.
 18. A camcorderon which a light-emitting device according to claim 10 is mounted.
 19. Avehicle body on which a light-emitting device according to claim 10 ismounted.
 20. An image forming apparatus for an electrophotographicsystem, wherein a light-emitting device according to claim 10 is presentin a display part of the image forming apparatus.
 21. An image formingapparatus for an electrophotographic system, wherein a light-emittingdevice according to claim 10 is mounted as an exposure light source fora photosensitive member of the image forming apparatus.
 22. A pluralityof passive driven light-emitting devices according to claim
 10. 23. Aplurality of active driven light-emitting devices according to claim 10.24. A plurality of light-emitting devices according to claim 23, whichemit monochromatic light.
 25. A plurality of light-emitting devicesaccording to claim 23, which emit lights different in color,respectively.
 26. A plurality of light-emitting devices according toclaim 25, which emit full-color light.
 27. A bottom emissionlight-emitting device according to claim
 10. 28. A top emissionlight-emitting device according to claim
 10. 29. A light-emitting deviceaccording to claim 10, which emits light having any one of three primarycolors of red, green and blue.
 30. A light-emitting device according toclaim 10, which emits light having a neutral color.